The crucial role of metal-zeolite interaction for stability of hybrid catalysts during CO2-to-DME hydrogenation

Introduction Dimethyl ether (DME) is considered a reliable alternative fuel, also being a key intermediate compound for the production of bulk chemicals, like olefins and gasoline-cut hydrocarbons [1]. Meeting the requirements for a full sequestration and recycling of carbon dioxide, it has been demonstrated that the synthesis of DME can be sustainably performed via CO2 hydrogenation directly in a one-step process [2]. In this perspective, a novel hybrid CuOZnO- ZrO2/ferrierite catalyst was recently proposed for one-pot hydrogenation of CO2-to-DME, exhibiting high DME productivity thanks to the unique shape-selectivity offered by ferrierite zeolite. However, a significant drop of activity after few hours of operation time (TR, 260°C; PR, 3.0 MPa; CO2/H2, 1/3 v/v) pushes now the research interest towards the development of more stable multifunctional systems, suitable to ensure activity, selectivity and lifetime under typical industrial conditions [3-6]. Materials and Methods In this work, the influence of home-made zeolite samples (i.e., Sil, MOR, MFI, FER, BEA, Y), integrated in a weight ratio of 1:1 with a CuO-ZnO-ZrO2 metal-oxide(s) phase, was investigated under long-term stability tests (PR, 3.0 MPa, TR, 260 °C, GHSV: 8,800 NL/h/gcat), in a fixed bed reactor (i.d., 4 mm) during CO2 hydrogenation reaction (CO2/H2/N2=3/9/1) to assess the activity-selectivity pattern of the hybrid catalyst as well as their deactivation trend during operation time. The hybrid multi-functional catalysts have been prepared by gel-oxalate co-precipitation of Cu-Zn-Zr (60/30/10 at.%) nitrate precursors in a slurry solution containing the zeolites of different topology. Structure, adsorption properties and surface reactivity of studied catalysts were investigated by BET, XRD, XPS, H2-TPR, N2O-titration and H2-CO2 chemisorption measurements. The acidity of bi-functional catalysts was evaluated by TPD of ammonia, pyridine adsorption and FTIR measurements. Results and Discussion What clearly appears from characterization data is that the nature of zeolite and its specific features directly affect not only the metallic properties, but also the acid capacity and distribution of acid sites in CuO-ZnO-ZrO2/zeolite catalysts. As regards the catalytic behaviour, notwithstanding a similar initial activity (XCO2?22%) of the investigated catalysts under the adopted experimental conditions, the obtained results highlighted how the extent of interaction between the metal-oxides phase with the zeolite surface leads to significant differences both in DME selectivity and catalyst lifetime. A linear relationship was found between the deactivation constants and the population of weak acid sites more prone to migrate in presence of water vapor formed during the reaction, also explaining the reduction in the total acidity observed after run. Therefore, if a suitable acid density is fundamental to drive the process towards the formation of DME, a lower framework density of the zeolite structure, such as in MOR or BEA, is also fundamental to realize a larger interface area with the metal-oxide sites, so leading to more stable catalysts (see Fig. 1). Significance The development of new processes based on CO2 utilization is a main current target in Europe, as evidenced from various EU H2020 calls and other initiatives on these topics. It is thus necessary to develop enhanced technology bases to contribute to climate change mitigation (in terms of CO2 avoidance) and for transition to a sustainable low-carbon economy.